The present invention relates to a pretensioner for a vehicle seat belt system and, in particular, to a pretensioner for tightening the belt by rotating the reel shaft of a belt retractor in the belt-winding direction.
Some vehicle seat belt systems have a pretensioner for tightening a belt worn by an occupant of the vehicle in order to tightly restrain the body of the occupant in an emergency and to prevent the occupant from undergoing secondary collision within the vehicle. Such a pretensioner is disclosed in Japanese Provisional Patent Publication No. 1-119454.
In the pretensioner of Japanese Publication No. 1-119454, a pulley connected to the reel shaft of a seat belt retractor by a coupling device is rotated, thereby to tighten the belt by a drive unit (power source) that pulls a tension cable wound on the periphery of the pulley. The coupling device comprises a gear fixed on the reel shaft and a gear fixed on the pulley. The pulley is supported on a guide unit (swing crank), and the guide unit is normally held in a "set" position in which the gears are disengaged by a holding unit (shear pin). When the drive unit is triggered, the holding unit releases the guide unit, which pivots to enable the gears to engage. After the gears are engaged, the holding unit releases the pulley so that it can rotate the reel shaft.
The operation of the holding unit is such that the rotation of the pulley must be restricted until the gears are engaged with each other, and this requires a holding unit having a high accuracy (specifically, a shear pin with an accurate shear load) that operates in two steps. Because a pretensioner must operate virtually instantaneously, in the order of a millisecond, even a slight change in resistance can greatly alter the operation of the unit. Since the pulley is supported by the guide unit (i.e., a swing crank) in the above device, a sturdy guide unit to match the operating power is needed, and there is a problem of an increase in power loss to operate the guide unit.